The present invention relates to a high-purity tantalum sputtering target having a uniform and fine structure and enabling plasma stabilization and achievement of superior film evenness (uniformity).
In recent years, the sputtering method for forming films from materials such as metal or ceramics has been used in numerous fields, which include the electronics field, the field of corrosion resistant materials and decoration, the catalytic field, as well as in the manufacture of cutting/polishing materials and abrasion-resistant materials.
While the sputtering method itself is a well-known method in the foregoing fields, particularly in the electronics field, a tantalum sputtering target suitable for forming films of complex shapes, forming circuits or forming barrier films is recently in demand.
Generally, this tantalum target is manufactured by repeating the hot forging and annealing (heat treatment) of an ingot or billet formed by performing electron beam melting and casting to a tantalum raw material, and thereafter performing rolling and finish processing (mechanical processing, polishing, etc.) thereto in order to process the ingot or billet into a target.
In this kind of production process, the hot forging performed to the ingot or billet will destroy the cast structure, disperse or eliminate the pores and segregations, and, by further performing annealing thereto, recrystallization will occur, and the densification and strength of the structure are improved.
The molten and cast ingot or billet generally has a crystal grain size of 50 mm or more. As a result of subjecting the ingot or billet to hot forging and recrystallization annealing, the cast structure is destroyed, and generally uniform and fine (100 μm or less) crystal grains can be obtained.
Meanwhile, if sputtering is to be performed using a target produced as described above, it is said that the recrystallized structure of the target becomes even finer and more uniform, more uniform deposition is possible with a target in which the crystal orientation is aligned toward a specific direction, and a film with low generation of arcing and particles and stable characteristics can be obtained.
Thus, measures are being taken, in the production process of the target, for achieving a finer and more uniform recrystallized structure and aligning the crystal orientation toward a specific direction (for example, refer to Patent Document 1 and Patent Document 2).
Moreover, disclosed is a high-purity Ta target for forming a TaN film to be used as a barrier layer against a Cu wiring film, obtained by using a high-purity Ta, in which an element having self-sustained discharge characteristics selected among Ag, Au and Cu is contained in an amount of 0.001 to 20 ppm, the total amount of Fe, Ni, Cr, Si, Al, Na, and K as impurity elements is 100 ppm or less, and the value after deduction of the content of these elements is within the range of 99.99 to 99.999% (refer to Patent Document 3).
When reviewing these Patent Documents, there is no disclosure to the effect that the inclusion of a specific element realizes a finer structure and thereby stabilizes the plasma.
In particular, Patent Document 3 describes that an element selected among Ag, Au and Cu is contained in an amount of 0.001 to 20 ppm, and the discharge amount of Ta ions increases by adding an infinitesimal amount, 0.001 ppm at minimum, of the element. However, since the additive element is contained in a trace amount, it is considered that there is a problem in that it is difficult to adjust the content and realize a uniform addition (spread).
In addition, as shown in Table 1 of Patent Document 3, the inclusion of Mo, W, Ge, and Co contents is respectively tolerated at less than 10 ppm, 20 ppm, 10 ppm, and 10 ppm. Accordingly, these impurities alone are contained in an amount less than 50 ppm.
Accordingly, as described above, Patent Document 3 describes that a high-purity Ta, in which the total amount of Fe, Ni, Cr, Si, Al, Na, and K as impurity elements is 100 ppm or less, and the value after deduction of the content of these elements is within the range of 99.99 to 99.999%, is used. However, the lower limit of the actual purity falls below (tolerates) 99.99%.
This is a level that is lower than conventional high-purity tantalum, and it is strongly assumed that the characteristics of high-purity tantalum cannot be utilized.
[Patent Document 1] Published Japanese Translation No. 2002-518593 of PCT Application
[Patent Document 2] U.S. Pat. No. 6,331,233
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2002-60934